The use of flowing electrochemical reactors, for example, in redox flow batteries and in various electrosynthesis processes, is increasing. This technology has the potential to be of central significance in the increased deployment of renewable electricity for carbon-neutral processes. A key element of optimizing efficiency of electrochemical reactors is the combination of high solution conductivity and reagent solubility. Here, we show a substantial rate of charge transfer for an electrochemical reaction occurring in a microemulsion containing electroactive material is loaded inside the nonpolar (toluene) subphase of the microemulsion. The measured rate constant translates to an exchange current density comparable to that in redox flow batteries. The rate could be controlled by the surfactant, which maintains partitioning of reactants and products by forming an interfacial region with ions in the aqueous phase in close proximity. The hypothesized mechanism is evocative of membrane-bound enzymatic reactions. Achieving sufficient rates of electrochemical reaction is the product of an effort designed to establish a reaction condition that meets the requirements of electrochemical reactors using microemulsions to realize a separation of conducting and reactive elements of the solution, opening a door to the broad use of microemulsions to effect controlled electrochemical reactions as steps in more complex processes.

Aqueous and non-aqueous redox flow batteries (RFBs) have limited energy and current densities, respectively, due to the nature of the electrolytes. New approaches to electrolyte design are needed to improve the performance of RFBs. In this work, we combined a highly conductive aqueous phase and an organic redox-active phase in a microemulsion to formulate a novel RFB electrolyte. As a proof-of-concept, we demonstrate an RFB using this microemulsion electrolyte with maximum current density of 17.5 mA·cm−2 with a 0.19 M posolyte and 0.09 M negolyte at a flow rate of only ∼2.5 ml·min−1, comparable to early vanadium electrolyte RFBs at similar flow rates on a per molar basis. The novel active negolyte component is an inexpensive oil-soluble vitamin (K3). By combining aqueous and organic phases, the solvent potential window and energy density may be increased without sacrificing current density and new redox couples may be accessed. Microemulsion electrolytes show great promise for improved performance and increased energy densities in aqueous RFBs but the path forward is complex. We end with discussion of areas that need work to achieve the potential of these electrolytes.

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Redox flow batteries have recently received considerable attention as possible large-scale energy storage devices, but their low energy density has inhibited widespread application. In this work, a novel strategy of decoupling conductivity and solubility of electrolytes using microemulsion is put forward to enhance ionic conduction of non-aqueous electrolytes, increase the selectivity of active species, improve the battery voltage, and eventually achieve the possibility of high energy density. We report a study of the electrochemistry of ferrocene in single phase Tween® 20/1-butanol/H2O/toluene microemulsion system at 20 °C. At low and intermediate surfactant to water weight ratios (<0.5/0.5), the voltammogram exhibits reversible electrochemical behavior, while at high surfactant levels the curves show lower levels of reversibility. The latter voltammograms have a form typically associated with high resistance in solution, consistent with a gradual transition in microstructure as surfactant levels increase. This change in structure is supported by correlations with conductivity results based on the literature. The voltammograms show little evidence of anomalies in double layer capacitance or electrode “blocking” by droplets, suggesting that the electron transfer is indeed occurring in a facile manner between the electrode and the ferrocene inside the oil phase.

Research and discovery of new electrolytes is crucial for advancing the fundamental understanding and development of new opportunities in electrochemical systems. Through an Energy Frontier Research Center (EFRC) for Breakthrough Electrolytes for Energy Storage (BEES), the approach we are taking is to establish a system for decoupling the nature and solubility of electroactive material from the conductivity and transport of ions in the surrounding solution. We accomplish this with microemulsions as a technique to solubilize hydrophobic redox active materials in an aqueous solvent. Microemulsions are isotropic, nanoscale dispersions of two immiscible liquids, stabilized by a surfactant, and cosurfactant. Microemulsions are an interesting electrolyte because they possess several properties that make them useful as an energy storage medium – they are thermodynamically stable, have fast dynamics, possess large interfacial areas, and can solubilize large concentrations of hydrophobic redox-active compounds in an aqueous electrolyte solution. Cyclic voltammetry experiments have shown reversible behavior in microemulsions up to 22 V/s with a double layer capacitance less than expected with such high scan rates. In order to understand the electrochemical behavior, we have begun with understanding the nanoscale structure of the microemulsion. Presented here are the small angle neutron scattering results on microemulsion systems with increasing surfactant loading and how the nanostructure correlates with the electrochemical response. That is as the strength of the amphiphile increases the maximum current density peak decreases and the half-wave potentials shift to lower potentials suggesting the local environment of the redox active species changes as the composition of the microemulsion is varied. Figure 1